Liquid delivery systems are widely used for dispensing fuels (such as gasoline and diesel fuel) and other liquids. As is well known to those having skill in the art, a liquid delivery system, such as a fuel delivery system, typically includes a submersible pump for pumping a liquid from a storage tank, through a liquid line, and to a dispenser. The submersible pump typically includes a pumphead located in a covered manway atop the storage tank, a pump turbine submersed in the liquid in the storage tank, and a tube connecting the pumphead to the pump turbine. The liquid line to the dispenser is typically connected to the pumphead in the covered manway.
An electrical power line is connected to the pumphead in the covered manway and extends from the pumphead to a submersible pump controller located outside of the covered manway. The submersible pump controller selectively supplies electrical power through the power line to thereby activate the submersible pump when dispensing of liquid is desired or at other predetermined times.
Often, the liquid being delivered in a liquid delivery system is toxic, highly volatile, flammable, or otherwise hazardous. Accordingly, it is both desirable and necessary to provide a leak detector for the liquid delivery system so that maintenance activities can be promptly instituted to correct any leaks. In fact, for fuel delivery systems, federal, state, and local environmental protection regulations often mandate leak detection systems. Some leak detection systems are adapted to detect a leak in the liquid storage tank. Other leak detection systems are adapted to detect leaks in the liquid line from the pumphead to the liquid dispenser.
The two primary concerns in the design of a line leak detection system are ease of installation of the system components and accuracy of leak detection. Ease of installation is important because the covered manway atop the storage tank is typically remote from the submersible pump controller. A thick concrete or asphalt slab is provided to isolate the underground storage tank from above ground activity and to provide a surface for vehicles. A dispenser, for example a gasoline pump, is typically mounted on the concrete slab above ground with the liquid line running underground from the pumphead in the covered manway to the liquid dispenser on the concrete slab. Also, the submersible pump controller is typically mounted in a gasoline service station building, separate from the gasoline dispenser, and is electrically connected to the pumphead via an electrical power line which runs under, or in a conduit through, the concrete slab. Accordingly, the design of a liquid leak detector should minimize the need to disrupt the concrete slab in order to install the liquid line leak detector.
Accuracy of line leak detection is also an important consideration in a line leak detector system. Accuracy is important because small leaks must be detected in order to shut down the submersible pump before an environmentally hazardous leak occurs. Repeated false leak alarms, caused by an inaccurate leak detection system, may cause an operator to ignore or bypass a true leak alarm. Moreover, accurate line leak detection is difficult for liquid hydrocarbon fuels because these fuels are subject to large transient volume contractions and expansions due to thermal variations. Thermal contractions caused by cooling of a fuel after it has been pumped into the fuel line result in a decrease in the volume of liquid within the liquid line which can appear to be a line leak. Accordingly, a line leak detector must distinguish between thermal contractions and a true leak.
The art has heretofore provided line leak detectors for liquid delivery systems. However, these line leak detectors have not been easy to install, particularly in retrofit applications. For example, U.S. Pat. No. 4,876,530 to Hill et al. discloses the use of a pressure transducer mounted separate from the system controller and requiring dedicated wiring to communicate with the system controller. Therefore, in a retrofit application, additional wiring must be placed between the controller in the service station building and the sensor in the covered manway atop the underground storage tank requiring the disruption of the concrete or asphalt slab.
Known line leak detectors have also not been highly accurate. For example, U.S. Pat. No. 4,518,955 to Meyer discloses a device for measuring leaks in piping by using a spring loaded piston located in line with the flow of liquid. The cylinder of the piston has two detection points to measure the axial displacement of the piston. These detection points can be either mechanical reed relays or Hall effect sensors so that the device can indicate one of three states: shut-off, leak, or normal flow. Unfortunately, measurement of gross axial displacement using sensors or relays only provides a gross measurement of fluid flow and does not provide an accurate indication of leaks. U.S. Pat. No. 4,088,987 to Resler et al. discloses the use of a pressure transducer installed on the line near the dispenser which triggers an alarm in response to a drop in pressure below a predetermined threshold, thereby being susceptible to inaccuracy caused by thermal contraction. Similarly, U.S. Pat. No. 4,876,530 to Hill et al. discloses the use of a spring loaded piston to pressurize a pipe from a make-up reservoir. A pressure transducer, separate from the spring loaded piston, is used to detect the decay in pressure over time which is indicative of a leak. However, the pressure transducer may provide an inaccurate indication of a line leak because thermal changes may cause pressure to fail to accurately correlate with the liquid volume loss caused by a leak.